The goal of this research is to find relationships between protease inhibitors participating in the regulation of enzymes involved in hemostasis. Studies are proposed to 1) analyze molecular structural relationships, and 2) compare the in vivo behaviors of the two major isoforms of antithrombin III, protein C ihibitor and alpha 2-antiplasmin. Each of these protease inhibitors is present in two distinct molecular forms. For antithrombin III a major difference exists between congeners in the regions binding heparin. Therefore, the structure of the heparin binding sites in the two forms of antithrombin III will be probed by modification and identification of amino acids known to participate in the heparin binding site, to determine if these residues vary between forms. Next, the heparin bindings sites of protein C inhibitor isoforms, and the plasminogen binding sites of alpha 2-antiplasmin will similarly be probed. After the development of suitable methods of purification and radioiodination have been developed, the in vivo kinetic behavior of the protein C inhibitor and alpha 2-antiplasmin isoforms will be investigated, using the rabbit as an animal model. Once the normal in vivo behavior has been determined in the animals, methods will be developed so these results can be verified in man. Turnover data will be carefully analyzed, as previously done for labeled-antithrombin III, to determine if these inhibitors also distribute to a non-circulating vascular (endothelial) pool.